In many different applications documents are provided which contain regions of raised material. Typically this is material that is either added to the document surface deliberately or as the result of some activity by a third party. It is often desirable to be able to detect such material, whether this material is, deliberately or accidentally, either attached to or forming part of the document. One particular application where such a procedure is important is in the field of the document security.
One such type of raised material is that of adhesive tape applied to documents such as banknotes. It is desirable to be able to detect the presence of such tape automatically since this may be indicative of damage to the document and the document (such as a banknote) can then be removed from circulation. Tape may also be present in counterfeit notes such as “composite notes”. One such method of detecting tape is provided in U.S. Pat. No. 4,525,630 in which photodetectors measure the level of light reflected specularly and diffusely from a region of the document, this comparison enabling adhesive tape to be detected due to its different reflective properties in comparison with those of the banknote itself.
However, there is a need to improve upon the detection of such raised material, not only in the terms of the type of material that may be detected, but also since counterfeiters and other persons interested in the breech of the document security, are increasingly sophisticated. There is therefore a desire to improve the ability to detect raised material on documents.